1. Field of Invention
This invention relates generally to laser systems, and their methods of use, that include harmonic generators, and more particularly to laser systems, and their method of use, that produce a constant UV output power with a minimal change in the UV beam parameters for an extended period of time
2. Description of the Related Art
Ultraviolet (UV) laser light is useful for many applications such as materials processing and inspection. A laser system for producing ultraviolet light can include a diode-pumped mode-locked solid-state laser to produce infrared light, and a harmonic generator to produce ultraviolet light. This type of laser system typically uses two nonlinear crystals in the harmonic generator. The first crystal converts the fundamental wavelength, in the infrared, to the second harmonic wavelength, in the green. The second crystal converts the fundamental and the second harmonic light to the third harmonic in the ultraviolet.
A significant problem with these laser systems is the limited lifetime of the nonlinear crystals, particularly the crystal that generates the UV. One example of this limited lifetime is given in xe2x80x9cHigh-power picosecond LiB3O5 optical parametric oscillators tunable in the blue spectral range,xe2x80x9d by B. Ruffing, A. Nebel and R. Wallenstein in Applied Physics B, 2000. Ruffing, et al., discloses that during operation there is a slow decrease of the power in time at a rate of about 5% per hour.
For many commercial applications, the power of the laser system needs to remain constant for thousands of hours. Additionally, several other properties of the beam must also remain constant during this time. In these applications, the beam of UV light typically passes through a series of lenses and modulators and then illuminates a sample. The size of the beam at each element is critical and should not change in time. Thus the UV beam parameters from the laser need to remain constant as well. The beam can be characterized by the size and location of the beam waist and the divergence of the beam. It is also convenient to calculate the M2 parameter of the beam from these numbers. The M2 parameter measures what multiple the beam is xe2x80x9ctimes diffraction limitedxe2x80x9d. It is desirable that the power of the beam, the beam waist location, the beam waist size, the beam divergence and the M2 parameter of the beam remain nearly constant for as long as possible.
If the power decreases due to crystal degradation, then the fundamental power from the laser can be increased. This keeps the UV power constant but does not compensate for an increase in the M2 parameter. Increasing the power of the fundamental can cause the beam waist size and location and the divergence of the UV beam to change even further.
The lifetime of the entire laser system can be extended by translating the crystal that generates the UV. To achieve the maximum lifetime from the crystal before it needs to be replaced, however, it is still necessary to obtain the longest lifetime from each individual spot.
There is a need for a laser system with a harmonic generator that can produce a constant UV output power with a minimal change in the UV beam parameters for an extended period of time.
Accordingly, an object of the invention is to provide a harmonic generator that can produce a constant UV output power for an extended period of time.
Another object of the invention is to provide a harmonic generator that can produce a UV output where the beam parameters remain constant for an extended period of time.
These and other objects of the present invention are achieved in a laser system with a high reflector and an output coupler that define a laser oscillator which produces a fundamental beam. A gain medium is positioned in the laser oscillator. A harmonic generator is positioned to receive at least a portion of the fundamental beam and produce a harmonic beam. The harmonic generator is configured to operate at a temperature selected to provide a least amount of change of a selected parameter of the harmonic beam for the longest period of time.
In another embodiment of the present invention, a method is provided for selecting a temperature of a harmonic generator included in a laser system that has a laser oscillator and a harmonic generator producing a harmonic beam. The laser system is operated at a constant harmonic beam power. A change in at least one beam parameter of the harmonic beam as a function of time for a plurality of temperatures is measured. A temperature is then selected that provides the least amount of change of a selected parameter of the harmonic beam for the longest period of time.